Electrophotographic type optical printers that use a laser light source are well known. More recently, however, optical printers using an LED array as the light source have been devised. Such LED array optical printers require a light-emitting apparatus for suitably modulating light emitted by the LED array, and exposing the target image on a photosensitive drum.
Referring now to FIG. 1, there is shown a general arrangement of a prior art light-emitting apparatus 10. In apparatus 10, an input board 12, an input connector 14, and a spreader board 16 are assembled on a motherboard 18. Light Emitting Diode (LED) array driver integrated circuits (IC) 20 and an LED array 22 are integrated on the motherboard 18. The LED array 22 comprises well over a hundred LEDs (not shown) arranged in a row. Wire bondings 24 are used to electrically connect (a) the input board 12 and the spreader board 16, (b) the spreader board 16 and the LED array driver ICs 20, and (c) the LED array driver ICs 20 and the LED array 22. A focussing rod-lens array (not shown) is disposed over the LED array 22 for focussing light from the LED array 22 onto a photosensitive drum (not shown in the optical printer).
Referring now to FIG. 2, there is shown a configuration of the LED array driver IC 20 of the prior art light-emitting apparatus 10 shown in FIG. 1. Each LED array driver IC 20 comprises a shift register 30, a latch circuit 34, a strobe circuit 35 comprising AND circuits 36, a drive circuit 37, a data input pad 38, a data output pad 39, a latch input pad 40, a strobe input pad 42, output pads 44 (three of which are labeled), and a clock input pad 46. The data input pad 38, data output pad 39, latch input pad 40, and strobe input pad 42 are connected to the spreader board 16 by the wire bondings 24 (shown only in FIG. 1), and the output pads 44 are connected to the LED array 22 by the wire bondings 24.
In the operation of the light-emitting apparatus 10, drive signal data (in serial form), clock signals, control signals, and power are inputted to the spreader board 16 via the input connector 14 and input board 12. Such power and other signals from the spreader board 16 are then inputted to the LED array driver IC 20 via the pads 38, 40, and 42. In the LED array driver IC 20, the serial drive (emission) signal data is converted to parallel form and used to drive LEDs 23 of the LED array 22 in accordance with the supplied control signals from the strobe input pad 42.
The drive data signals coming from the spreader board 16 are inputted to the shift register 30 via the data input pad 38. In the shift register 30, the drive data is shifted from left to right in accordance with clock signals inputted from the spreader board 16 via the clock input pad 46. From the shift register 30, the drive data signals thus shifted to the right are outputted from the data output pad 39 and, via the spreader board 16, are inputted to the data input pad 38 of the next LED array driver IC 20 to the right. When one entire line of print signal data is thus prepared representing on/off data for each LED, it is latched by the latch circuit 34 in accordance with control signals inputted via the latch input pad 40.
An AND operation at each AND gate 36 is then applied to the associated (a) latched data signals inputted to the strobe circuit 35 via the latch circuit 34, and (b) strobe control signals inputted to the strobe circuit 35 via the strobe input pad 42, to thereby determine an LED emission exposure time. Exposure signals outputted along each of the channels from the AND circuits 36 by the strobe circuit 35 are converted by the drive circuit 37 to prescribed drive currents used to operate the respective LEDs 23 of the LED array 22.
To energize all of the LEDs 23, it is necessary to operate all of the strobe circuit channels simultaneously. This causes the type of large current pulses graphically shown in FIG. 3. If each LED draws 5 mA, and the apparatus 10 has a total of 5,000 LEDs 23, the result is a square-shaped 25-amp pulse. Such pulses generate wide-ranging electromagnetic radiation interference (EMI) in the signal and power supply (not shown) which degrades print quality.
A further problem is that, owing to the inductance and resistance components in power supply lines of the driver array ICs 20, the wiring of the spreader board 16, and the power cable connected to the apparatus 10, large pulses of current such as these result in a decrease in the voltage going to the array driver ICs 20, producing both variations in optical output and degraded print quality.